The present invention pertains to organic electroluminescent devices, such as discrete light emitting devices, arrays, displays, and in particular to injection layers and contact electrodes suited for such devices. It furthermore relates to a method for making the same.
Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting devices, arrays and displays. Organic materials investigated so far can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The ease of fabrication and extremely high degrees of freedom in organic EL device synthesis promises even more efficient and durable materials in the near future which can capitalize on further improvements in device architecture.
Organic EL at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al., Journal Chem. Phys., Vol. 38, 1963, pp. 2024, and in xe2x80x9cRecombination Radiation in Anthracene Crystalsxe2x80x9d, Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred largely by two reports of high efficiency organic EL. These are C. W. Tang et al, xe2x80x9cOrganic electroluminescent diodesxe2x80x9d, Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and by a group from Cambridge University in Burroughs et al., Nature, Vol. 347, 1990, pp. 539. Tang et al. made two-layer organic light emitting devices using vacuum deposited molecular dye compounds, while Burroughs used spin coated poly(p-phenylenevinylene) (PPV), a polymer.
The advances described by Tang and in subsequent work by the Cambridge group, for example in xe2x80x9cEfficient LEDs based on polymers with high electron affinitiesxe2x80x9d, N. Greenham et al., Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in the design of EL devices derived from the selection of appropriate organic multilayers and contact metals.
Organic EL light emitting devices (OLEDs) function much like inorganic LEDs, except that light is commonly extracted through a transparent electrode deposited on a transparent glass substrate. The simplest possible structure, schematically illustrated in FIG. 1A, consists of an organic emission layer 10 sandwiched between two electrodes 11 and 12 which inject electrons (exe2x88x92) and holes (hxe2x80x2), respectively. Such a structure has been described in the above mentioned paper of Burroughs et al., for example. The electrons and holes meet in the organic layer 10 and recombine producing light. It has been shown in many laboratories, see for example: xe2x80x9cConjugated polymer electroluminescencexe2x80x9d, D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401-405, xe2x80x9cThe effect of a metal electrode on the electroluminescence of Poly(p-phenylvinylene)xe2x80x9d, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35, No. 3 A, 1996, pp. 1317-1319, and xe2x80x9cCarrier tunneling and device characteristics in polymer LEDsxe2x80x9d, I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666, that improved performance can be achieved when the electrode materials are chosen to match the respective molecular orbitals of the organic material forming the organic layer 10. Such an improved structure is shown in FIG. 1B. By choosing the optimized electrode materials 13 and 14, the energy barriers to injection of carriers are reduced, as illustrated. Still, such simple structures perform poorly because little stops electrons from traversing the organic layer 10 and reaching the anode 14, or the hole from reaching the cathode 13. FIG. 2A illustrates a device with a large electron barrier 16 such that only few electrons are injected, leaving the holes no option but to recombine in the cathode 15.
A second problem, illustrated in FIG. 2B, is that the mobilities of electrons and holes in most known organic materials, especially conductive ones, differ strongly. FIG. 2B illustrates an example where holes injected from the anode 18 quickly traverse the organic layer 19, while the injected electrons move much slower, resulting in recombination near the cathode 17. If the electron mobility in the organic layer 19 were larger than the holesxe2x80x2, recombination would occur near the anode 18. Recombination near a metal contact is strongly quenched by the contact which limits the efficiency of such flawed devices.
Tang, as shown in FIG. 3, separated electron and hole transport functions between separate organic layers, an electron transport layer 20 (ETL) and a hole transport layer (HTL) 21, mainly to overcome the problems described above. In xe2x80x9cElectroluminescence of doped organic thin filmsxe2x80x9d, C. W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989, pp. 3610-3616, it is described that higher carrier mobility was achieved in the two-layer design, which led to reduced device series resistance enabling equal light output at lower operating voltage. The contact metals 22, 23 could be chosen individually to match to the ETL 20 and HTL 21 molecular orbitals, respectively, while recombination occurred at the interface 24 between the organic layers 20 and 21, far from either electrode 22, 23. As electrodes, Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO) as the anode. Egusa et al. in xe2x80x9cCarrier injection characteristics of organic electroluminescent devicesxe2x80x9d, Japanese Journal of Applied Physics, Vol. 33, No. 5 A, 1994, pp. 2741-2745 have shown experimentally that the proper selection of the organic multilayer can lead to a blocking of both electrons and holes at an organic interface remote from either electrode. This effect is illustrated by the structure of FIG. 3 which blocks electrons from entering the HTL 21 and vice versa by a clever choice of HTL and ETL materials. This feature eliminates non-radiative recombination at the metal contacts as illustrated in FIG. 1A and also promotes a high density of electrons and holes in the same volume leading to enhanced radiative recombination.
The heterojunction molecular orbital energy alignment illustrated in FIG. 3 actually reflects a trend in preferred OLED materials which is beneficial to device design (and to the present invention as will be discussed later). The trend is that materials which tend to transport electrons with high mobility 20 do so, in part, because their LUMOs lie at lower energy. Similarly, good hole conductive properties go hand-in-hand with HOMOs lying at higher energy. These facts make it more probable that a heterojunction formed between an electron and hole transporting organic layer will block the injection of one or both carriers at the interface due to the energy discontinuities of the respective molecular orbitals. The blocking effect localizes the carriers far from the quenching electrodes where they can recombine most efficiently.
Two technical terms are commonly used which describe the positioning of the two relevant organic molecular orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of organic materials, or in the case of semiconductors, the positioning of their respective counterparts, the valence bands (VB) and conduction bands (CB). These terms in our treatment all have units of lower energies than a free electron, which reflects the fact that they are bound, and energy is required to remove electrons from nearly all known materials. For convenience, we arbitrarily define a free electron to have zero energy, and therefore speak in terms of the above quantities having negative energy values with respect to the free electron (or vacuum) state. The first of these terms is the work function, which describes how much energy is required to xe2x80x98pullxe2x80x99 the most weakly bound electron out of the material, i.e. to make it a free electron. In a metal or degenerate semiconductor (i.e. a semiconductor characterized by an extremely high free carrier concentration), the work function is identical to another quantity called the Fermi energy level. Metals or degenerate semiconductors have partially filled bands which are responsible for their large number of free conduction carriers. The Fermi energy level is the energy to which the highest energy band is filled, because it is these highest energy electrons which are the easiest to liberate energetically.
In the following, whenever we refer to an energy level, be it the work function, HOMO or LUMO, conduction band or valence band, or Fermi energy, it is in the above context.
With multilayer device architectures now well understood and commonly used, the major performance limitation of OLEDs is the lack of ideal contact electrodes. The main figure of merit for electrode materials known in the art is the position in energy of the electrode Fermi level relative to the energy of the organic molecular orbital into which it must inject (see Bradley and Parker above for detailed discussion). In some applications it is also desirable for the electrode material to be either transparent or highly reflective. The electrode should also be chemically inert and capable of forming a dense uniform film to effectively encapsulate the OLED. It is also desirable that the electrode not strongly quench organic EL.
No cathode material has yet been identified which is transparent, conductive, chemically stable, and a good electron injector for OLEDs. Good electron transporting organic materials have their lowest unoccupied molecular orbitals (LUMO) matched in energy only with low work function metals. A low work function in a metal is tantamount to high chemical reactivity. While, e.g., Ca has its work function well matched in energy to an Alq3 (tris(8 -hydroxyquinoline aluminum)) organic electron transport layer LUMO, a Ca cathode survives intact only a short time in air, leading to rapid device degradation. It is also likely that such highly reactive metals undergo a chemical reaction with the nearby organic materials which also could have negative effects on device performance. Such a mechanism has been proposed by Parker in the above cited reference to explain why Sm or Yb cathode OLEDs have poorer performance than Ca cathode OLEDs despite the lower work function of Sm and Yb compared to Ca. A low work function cathode metal approach requires careful handling of the device to avoid contamination of the cathode metal, and immediate, high quality encapsulation of the device if operation in a normal atmosphere is desired. Even well encapsulated low work function metal contacts are subject to degradation resulting from naturally evolved gases, impurities, solvents from the organic LED materials.
On the other hand, the choice of a stable metal having a higher work function, e.g. Al, dictates that the device can only be operated at high voltages. High voltage is necessary because electron injection from Al into, e.g., the Alq3 LUMO is field assisted. The high operating voltage reduces device efficiency due to increased ohmic losses. In addition, the higher electrical fields present at increased voltages also are likely to degrade the device materials more rapidly by driving interdiffusion or exciting parasitic chemical reactions or recombination processes. Al contacts, of lesser reactivity compared to Mg or Ca, have still been observed to degrade during OLED operation, see e.g. L. M. Do et al., xe2x80x9cObservation of degradation processes of Al electrodes in organic EL devices by electroluminescence microscopy, atomic force microscopy, and Auger electron microscopyxe2x80x9d, Journal of Applied Physics, Vol. 76, No. 9, 1994, pp. 5118-5121.
Many approaches have been attempted in order to solve the problem of cathode electrode instability, degradation and high injection voltage. A common approach is the use of a low work function metal subsequently buried under a thicker metal coating. In this case, pinholes in the metal still provide ample pathways for oxygen and water to reach the reactive metal below, as is described in: Y. Sato et al., xe2x80x9cStability of organic electroluminescent diodesxe2x80x9d, Molecular Crystals and Liquid Crystals, Vol. 253, 1994, pp. 143-150 and J. Kido et al., xe2x80x9cBright organic electroluminescent devices with double-layer cathodexe2x80x9d, IEEE Transactions on Electron Devices, Vol. 40, No. 7, 1993, pp. 1342-1344. Furthermore, such contacts are degraded by evolved gases from the OLED constituent materials. The overall lifetime of OLEDs using this approach is limited and extensive encapsulation is required.
Much less attention has been paid to the optimization of the anode contact, since ITO or Au anodes generally outperform the cathode contact. However, if the anode electrode could be improved, it would have a similarly positive effect on device performance and reliability as improved cathodes.
Indium-tin-oxide has been the anode of choice for years. Its major advantage is that it is a transparent conductor which also has a large work function (roughly 4.9 eV), and is therefore well suited for the formation of a transparent anode on glass. However, ITO is known to have a barrier to hole injection into preferred organic HTL materials. Parker showed that, by replacing ITO with Au, which has a larger work function, in an identical OLED structure, the device efficiency is doubled due to the elimination of the ITO/organic hole injection barrier. ITO is also responsible for device degradation as a result of In diffusion emanating from the ITO into the OLED which can eventually cause short circuiting. In diffusion from ITO into PPV was clearly identified in G. Sauer et al., xe2x80x9cCharacterization of polymeric LEDs by SIMS depth profiling analysis,xe2x80x9d Fresenius J. Analyt. Chem., in press. ITO also acts as an reservoir of oxygen, which is detrimental to organic LED materials when it diffuses from the ITO into the organic layers. This problem has been elucidated in: J. C. Scott et al., xe2x80x9cDegradation and failure of MEH-PPV light-emitting diodesxe2x80x9d, Journal of Applied Physics, Vol. 79, 1996, pp. 2745-2751. ITO is a polycrystalline material in the form commonly used for OLEDs. The abundance of grain boundaries provides ample pathways for contaminant diffusion through the ITO. Finally, ITO also is a reservoir of oxygen which is known to have a detrimental effect on common organic materials. Despite all of these known problems related to ITO anodes, they are still favored in the art because no other transparent electrode material of similar or better quality is yet known in the art. At least one transparent electrode is necessary for a practical OLED, since the light must be efficiently extracted to be useful.
European Patent application 448,268 concerns a semiconductor luminescent device with an organic layer being sandwiched between two electrodes. At least one of these electrodes consist of an n-type inorganic semiconductor. A junction between this electrode and the organic layer forms a blocking contact against electron injection.
The article xe2x80x9cSilicon Compatible Organic Light Emitting Diodexe2x80x9d, H. Q. Kim et al., Journal of Light wave Technology, Vol. 12, No. 12, December 1994, pp. 2107-2112, concerns organic light emitting diodes using n-doped silicon as cathode, or p-doped silicon as anode.
While Au has a large (5.2 eV) work function, long-lived OLED devices cannot be made using Au cathodes because of the very high diffusivity of Au in organic materials. Like In and O diffusion out of ITO, only worse, Au from the contact diffuses through the OLED and eventually short circuits the device. In addition, Au is not a practical anode material for most architectures because it is not transparent. For the lack of a transparent cathode material, the anode must be the transparent contact for present day OLEDs.
Other semiconductors besides ITO have been tried as OLED electrodes. I. D. Parker and H. H. Kim, xe2x80x9cFabrication of polymer light-emitting diodes using doped silicon substratesxe2x80x9d, Applied Physics Letters, Vol. 64, No. 14, 1994, pp. 1774-1776, showed that, depending on the semiconductor doping, the Si/SiO2 is capable of either hole or electron injection into organic thin films. This work applied the Si semiconductor electrode towards majority carrier injection, i.e. n-type Si for electron injection and p-type for hole injection. Si electrodes had a large barrier to both electron and hole injection into OLED materials. This is due to the small bandgap of Si and the moderate positioning of the Si conduction and valence bands in energy. Si is also absorbing to much of the visible spectrum, and represented no improvement over conventional metals. Parker and Kim circumvented the poor Si band energy positioning by adding a SiO2 interlayer between the Si contact and OLED. While the voltage drop across the SiO2 insulator permitted the Si bands to line up with their organic molecular orbital counterpart, electrons were not directly injected, rather forced to tunnel through the SiO2 insulator. Such OLEDs had turn-on voltages of  greater than 10 V, too high for efficient device operation.
The lack of inert, stable, energetically matched, and transparent electrode materials for low voltage, efficient and stable OLED operation remains a major obstacle to OLED development.
Organic LEDs have great potential to outperform conventional inorganic LEDs in many applications. One important advantage of OLEDs and devices based thereon is the price since they can be deposited on large, inexpensive glass substrates, or a wide range of other inexpensive transparent, semitransparent or even opaque crystalline or non-crystalline substrates at low temperature, rather than on expensive crystalline substrates of limited area at comparatively higher growth temperatures (as is the case for inorganic LEDs). The substrates may even be flexible enabling pliant OLEDs and new types of displays. To date, the performance of OLEDs and devices based thereon is inferior to inorganic ones for several reasons:
1. High operating voltage: Organic devices require more voltage to inject and transport the charge to the active region (emission layer) which in turn lowers the power efficiency of such devices. High voltage results from the need for high electric fields to inject carriers over energy barriers at the electrode/organic interfaces, and from the low mobility of the carriers in the organic transport layers (ETL and HTL) which leads to a large ohmic voltage drop arid power dissipation.
2. Low brightness: Today""s OLEDs can produce nearly as many photons per electron as common inorganic LEDs, i.e. their quantum efficiency is good. OLEDs lag inorganic LEDs in brightness mainly because comparatively little charge can be conducted through the resistive transport layers (HTL or ETL). This well known effect is referred to as Space Charge Limited Current. Simply put, due to the low mobility of carriers in organic materials, a traffic jam develops which restricts the flux of electrons and holes reaching the emission layer. Better emitter materials cannot offer greatly improved brightness until high conductance transport layers are also available.
3. Reliability: Organic LEDs degrade in air and during operation. Several problems are known to contribute.
A) Efficient low field electron injection requires low work function cathode metals like Mg, Ca, Li etc. which are all highly reactive in oxygen and water. Ambient gases and impurities coming out of the organic materials degrade the contacts.
B) Conventional AgMg and ITO contacts still have a significant barrier to carrier injection in preferred ETL and HTL materials, respectively. Therefore, a high electric field is needed to produce significant injection current. Stress from the high field and ohmic heating at the resistive electrode/organic interface contribute to device degradation.
C) The high resistance of carrier transport layers heats the device under operation.
D) Thermal stability of most OLED materials is poor making them sensitive to heating. Upon heating, many amorphous organic materials crystallize into grains. The crystallites have less volume and pack less uniformly than the amorphous solid. The resulting gaps and odd shapes of the crystallites make conduction from one crystallite to the next difficult, increasing resistance and heating in a positive feedback loop, while opening further channels for gaseous contaminants to penetrate, or for neighboring materials to diffuse. The relationship between crystallization in organic materials and the mobility is well understood from the photoconductor literature in for example: Borsenberger and Weiss, xe2x80x9cOrganic photoreceptors for imaging systemsxe2x80x9d, Marcel Dekker Inc., New York, 1993.
4. Poor chemical stability: Organic materials commonly used in OLEDs are vulnerable to degradation caused by reaction with and diffusion of contact electrode materials and the ambient atmosphere.
OLEDs are mainly limited by their contacts and transport layers, and feedback from the transport layer heating. It is thus highly desirable to replace the low work function metal based cathodes with a stable, possibly transparent cathode characterized by barrierless charge injection into OLEDs. It is also highly desirable to replace ITO anodes with a stable, non-diffusive, and possibly transparent anode characterized by barrierless charge injection into OLEDs.
However, present day solutions inhibit performance and degrade device reliability. The price of distancing the active layer from the metal contacts for higher recombination efficiency are ohmic voltage drops across the HTL/ETL, leading to heating and power consumption. Low work function metals are unstable and unreliable. ITO introduces impurities and has a barrier to hole injection.
As can be seen from the above examples and the description of the state of the art the contact materials need to be improved to realize OLEDs and displays based thereon with superior characteristics. Little progress has been recorded in the search for improved electrode materials, because researchers have only searched within the known paradigm of what defines a good electrode material: a material having a favorable work function.
The work function of a material is defined as the separation in energy between the Fermi energy and the vacuum reference energy. In metals, one can inject electrons from just below the Fermi energy. or holes from just above the Fermi energy. There is no possibility of using other bands due to the density of electrons. Although ITO is theoretically a wide bandgap semiconductor, it corresponds to the classical metal-based model, as we discuss below.
ITO is a wide bandgap semiconductor which has been successfully used to inject holes into OLEDs. ITO is a highly degenerate n-type material characterized by electron concentrations on the order of 1021 cmxe2x88x923. The ITO conduction band is positioned at approximately the correct energy for injecting holes into an organic HOMO, i.e. ITO has a large work function. Because of the large electron concentration, the ITO Fermi energy, which defines the work function for a given ITO sample, lies several 100 meV above the conduction band. Above the Fermi level are empty states which act as holes, and it is these holes, not ones in the VB, which are injected into the organic material. Therefore, ITO electrodes inject via the exact mechanism that a Au electrode does, from just above the Fermi energy, and ITO does not fall under the inventive approach described below.
It is an object of the present invention to provide new and improved organic EL devices, arrays and displays based thereon.
It is a further object of the present invention to provide new and improved organic EL devices, arrays and displays based thereon with improved efficiency, lower operating voltage, and increased stability and reliability.
It is a further object to provide a method for making the present new and improved organic EL devices, arrays and displays.
The above objects have been accomplished by providing an OLED wherein at least one of the contact electrodes, either the cathode or anode, comprises a non-degenerate wide bandgap semiconductor (n-d WBS), i.e., a semiconductor having a bandgap greater than 2.5 eV. If the anode comprises a n-d WBS, this n-d WBS is to be chosen such that holes are injected from the valence band of the anode into the HOMO of the adjacent organic material. A n-d WBS cathode has to be chosen such that electrons are injected from the conduction band of the n-d WBS into the LUMO of the adjacent organic material.
The inventive approach depends on the fact that any semiconductor whose bandgap is comparable or greater than the bandgap of typical OLED materials, i.e.  greater than 2.5 eV, will xe2x80x98a priorixe2x80x99 have its conduction and/or valence band positioned at a favorable energy level with respect to the organic HOMO or LUMO, respectively, such that injection of one or both carrier types can occur at low voltage across little or no energy barrier. The inventive approach also benefits from the many improved properties of semiconductors, especially non-degenerate wide bandgap semiconductors, for OLED electrodes, including good conductivity, transparency in the visible spectrum, chemical inertness, hardness, and ability to be deposited in the amorphous or polycrystalline state at extremely low temperatures on glass, organic thin films, or other amorphous or crystalline substrates. Plastic may also serve as substrate.
In one embodiment of the present invention, a single or multi-layer OLED structure having a n-d WBS cathode directly in contact with the corresponding organic layers, and a conventional opposite contact electrode is envisioned.
In another embodiment of the present invention, a single or multi-layer OLED structure having a n-d WBS anode directly in contact with the corresponding organic layer, and a conventional opposite contact electrode is envisioned.
In another embodiment of the present invention, a single or multi-layer OLED structure having both a n-d WBS anode and a n-d WBS cathode directly in contact with the corresponding organic layer is envisioned.
In another embodiment of the present invention, an OLED structure having a n-d WBS electrode whose performance is improved by introducing a second and/or third semiconductor is envisioned. The second semiconductor is in direct contact with the corresponding organic layer, and is characterized by an improved matching of the injecting band to the corresponding organic layer molecular orbital. The second semiconductor might be an alloy of the n-d WBS, or it might be a wholly different semiconductor. The third semiconductor is farthest from the organic layers, and is characterized by the ability lo form an improved ohmic contact to a highly conducting lateral transport layer. The third semiconductor might also be an alloy of the n-d WBS, or it may be a wholly different semiconductor.
In yet another embodiment of the present invention, an OLED in which a n-d WBS injecting layer is in direct contact to the nearest organic layer, but has a thin embedded metal interlayer very near to the n-d WBS/organic interface. The metal can be selected for its work function, properties as a diffusion barrier between the organic materials and the n-d WBS, or transparency, and serves the purpose of further improving the stability or electron injection of the n-d WBS/organic interface. The n-d WBS on the opposite side of the thin embedded metal layer can be the same n-d WBS, or a different n-d WBS.
In yet another embodiment of the present invention, an OLED in which a n-d WBS electrode is separated from the nearest organic layer by a thin metal interlayer is envisioned. The metal can be selected for its transparency, work function, or properties as a barrier between the organic materials and n-d WBS, and serves the purpose of further improving the stability or electron injection of the n-d WBS/organic interface.
The introduction of a n-d WBS based-electrode leads to the following advantages:
1. Low voltage carrier injection is realized through the highly favorable alignment of the n-d WBS energies with respect to preferred OLED materials.
2. n-d WBS""s are highly transparent to visible light.
3. n-d WBS""s are chemically inert and thermally stable and therefore have no undesirable solid state interactions with the organic layers with which it is in contact or close proximity.
4. n-d WBS""s are an outstanding encapsulant and mechanical protectant material for OLEDs, due to their nearly amorphous state and low impurity diffusion rates.
5. n-d WBS""s can be deposited at conditions required for OLED formation (e.g. low temperature, amorphous substrates, minimum damage to the growth surface) in a conductive state.
6. n-d WBS""s quench optical recombination in nearby organic layers less strongly than metals enabling reduced transport layer thicknesses.